Diffuse reflectance spectroscopy (DRS) has been widely used as a non-invasive tool for obtaining quantitative information about the optical properties of in-vivo tissue (e.g., absorption and scattering properties of biological molecules in tissue). In a DRS system, a light beam is directed into a target specimen (such as a biological tissue) and scattered within the specimen. Light diffusely reflected from within the specimen is then captured to obtain a reflectance spectrum thereof. The reflectance spectrum is then combined with light propagation models to determine the optical properties (e.g., absorption and scattering properties) of the specimen. The acquired optical properties can then be related to the composition, structure and microstructure of the specimen.
As shown in FIG. 1, a DRS system may use one of the following light sources: (1) a steady light source: emitting a light with constant intensity into the specimen, shown in FIG. 1(a); (2) a pulsed light source: emitting a modulated time domain light into the specimen, shown in FIGS. 1(b); and (3) a modulated frequency light source: emitting a light with modulated sine wave frequency into the specimen, shown in FIG. 1(c).
FIG. 2 shows a conventional DRS system 200 with multiple source-detector separations. The DRS system 200 comprises a light emitting unit 201 and two light detecting units 202a and 202b. The light emitting unit 201 and the two light detecting units 202a and 202b are placed on adjacent spots of a surface of a specimen 203. The light emitting unit 201 and the light detecting unit 202a are separated by a distance D1 while the light emitting unit 201 and the light detecting unit 202b are separated by a distance D2 (which is greater than D1). Since the source-detector separations D1 and D2 are different, the reflectance spectrum measured at light detecting unit 202a will be different from the reflectance spectrum measured at light detecting unit 202b, as a distinction can be made between light that has traveled shorter distances through the target specimen (low penetration, less interaction with the target specimen) and light that has propagated through the target specimen for longer times and distances (penetrated deeper, more interaction with the target specimen). Therefore, the two reflectance spectra can be analyzed and combined to obtain sufficient information for the separation of the scattering and absorbance properties/coefficients.
Nevertheless, a conventional DRS system with multiple source-detector separations has the following drawbacks. Firstly, the source-detector separation must be large enough for accurate measurement. This makes it extremely difficult to reduce the size of the DRS system. Also, having multiple source-detector separations means that some source-detector separations are larger than other source-detector separations, and larger source-detector separation implies that, when taking measurements, light diffusely reflected from deeper (underlying) layers of a target specimen cannot be avoided. That is, the light received at shorter distances (D1) only contains information on the shallow layer, while at longer distances (D2) the collected light is influenced by the shallow layer as well as the underlying layers. Therefore, it is not practical to use a DRS system with multiple source-detector separations to determine the optical properties of the shallow layers of a target specimen. Moreover, for a DRS system with multiple source-detector separations, due to inhomogeneity in optical properties within the target specimen, diffuse reflectance taken by different detectors cannot be easily calibrated for the system response using homogeneous reference tissue simulating phantom with known optical properties. Furthermore, the conventional DRS system 200 needs at least three optical fibers: one for the light emitting unit 201 and at least two for the light detecting units 202a and 202b. Moreover, an optical switch (not shown) is required for the conventional DRS system 200 to switch between the light detecting units 202a and 202b. Therefore, a novel DRS system is still needed to overcome the aforementioned drawbacks.